The Ack/Nack (acknowledgment/negative acknowledgement) fields in the R-ACKCH (reverse link acknowledgement channel) in CDMA2001x EV-DV or UL HS-DPCCH (up link high speed—dedicated physical control channel) in UMTS are error controlled independently by repetition block code. In CDMA2001x EV-DV, the R-ACKCH contains 1-bit Ack/Nack per 1.25 ms PCG (power control group) with symbol repetition of 24 times. In UMTS, the acknowledgement bit is repetition coded to 10 bits and transmits in one slot. An Ack sent in response to a received transmission indicates the transmission was properly received. A Nack in response to a transmission indicates the transmission was not properly received. In response to an Ack, the transmitter transmits the next data for transmission. In response to a Nack, however, the transmitter retransmits the Nacked transmission or other redundancy version depending on the HARQ protocol type. The Ack/Nack fields in EV-DV and HSDPA are both DTX (i.e., transmitter power set at zero) when no Ack/Nack data is transmitted. Since the R-ACKCH and HS-PDCCH field are not error free, it is important to evaluate the reliability of each received Ack/Nack information.
To improve reliability, a three-state decoding mechanism is included in the rake receiver design of channel elements to support EV-DV and HSDPA (high speed data packet access) for UMTS. The three state decoding mechanism adds a new state of erasure to erase any unreliable feedback information. The unreliable feedback information is caused by information corruption through unreliable radio channel or no Ack/Nack being sent because of the missed detection of the DL (down link) high speed data. Currently, two thresholds are set to define whether feedback information qualifies as an Ack, a Nack, or an erasure. When the power level of the received feedback information falls below the first threshold, the feedback information is judged to be an Ack. When the feedback information is greater than a second threshold, which is greater than the first threshold, the feedback information is judged to be a Nack. And, when the power of the feedback information falls between the first and second thresholds, inclusive, the feedback information undergoes erasure. Erasure results in considering the erased feedback information as a Nack.
The merit of the three-state decoding strategy is to erase any unreliable feedback information to minimize throughput damaging errors. For example, when a Nack is decoded as an Ack, the receiver losses data, and must make a separate request for retransmission. And, if an Ack is considered a Nack, then needless retransmission of data takes place. Both of these errors will impact the system throughput. The three-state decoding strategy is to erase any unreliable feedback information rather than to pass it to the HARQ protocol. This particularly helps mitigate against the more damaging throughput error of considering a Nack as an Ack. However, the erasure of the feedback information has the setback of loosing the desired feedback information, and the consequence of the Ack/Nack erasure is the re-transmission of the downlink high-speed data information at the HARQ. Thus, it is important to determine how to properly erase the feedback information. As will be appreciated, this involves properly determining the thresholds for the Ack/Nack.